1. Field of the Invention
The present invention relates to a laser peening apparatus.
2. Description of Related Art
As a method for giving a residual stress to a surface of a mechanical part or the like to reform the surface, a laser peening method is used.
Generally, working by a laser peening method is carried out in such a manner that a to-be worked surface is irradiated with a laser beam in a focusing manner, generated plasma is trapped in liquid inertial medium such as water and oil formed on the to-be worked surface, and a shock wave pressure generated thereby is given to the to-be worked surface. To effectively give the shock wave pressure produced by plasma to the to-be worked surface, it is necessary to form liquid such as water and oil for trapping the plasma on the to-be worked surface.
Japanese Patent Application Laid-open Publication No. 2000-246468 (hereinafter referred to as “Patent Document 1”) and Japanese Patent Application Laid-open Publication No. 2006-122969 (hereinafter referred to as “Patent Document 2”) describe laser peening methods in which a water film or other liquid film is formed on a to-be worked surface, and plasma is trapped by this liquid film. According to these publications, a protection film such as black paint is previously applied to the to-be worked surface. It is described in Patent Document 2, paragraph 0013 that running water is used as the water film.
Japanese Patent Application Laid-open Publication No. 2002-346847 (hereinafter referred to as “Patent Document 3”) describes a laser peening method carried out by disposing a laser irradiation head and a workpiece in the water (see Patent Document 3, paragraph 0004, FIGS. 10 and 11).
Patent Document 3 also describes a peening method using water jet and laser in combination (see Patent Document 3, claims and FIG. 1).
However, the above techniques have the following problems.
In the laser peening method in which the laser irradiation head and the workpiece are disposed in the water as described in Patent Document 3, there is a problem that a water tank which is large enough to accept an apparatus immersible portion such as the laser irradiation head and the workpiece is required.
Irrespective of water film type, in-water type and combination type of water jet and laser, if water is used, there is a problem of corrosion, and if other liquid is used, there is a problem of contamination depending upon the kind of the liquid.
There is also a problem that liquid for trapping plasma is consumed in high volume for storing water in the water tank and for running water for forming a water film.
In the peening method using the water jet and laser in combination described in Patent Document 3, bubble pressure crushing effect is dispersed in the radial direction by fluid effect of the water jet, and although an effective area is increased, permeation depth of a residual stress becomes shallow (see Patent Document 3, paragraph 0021), an original deep working by laser peening can not be carried out.